1. Field of the Invention
This invention relates generally to gas and pH measurement technology, and more particularly concerns an automated system for controlling the flow of gases for tonometric calibration of biomedical blood gas and pH sensors.
2. Description of Related Art
Accurate measurement of pH and the partial pressures of carbon dioxide and oxygen in the blood have become important in modern medicine, particularly for monitoring the respiratory condition of a patient. Extremely small multiparameter optical sensors have been developed for taking intravascular measurements of acidity, carbon dioxide and oxygen levels in the blood. Such sensors allow remote measurement of these parameters when combined with compatible intravascular catheter systems. Onsite calibration of such sensors before use in a medical setting is important, and it is desirable to provide automated systems for doing so.
Conventional systems for automatic tonometric calibration of gas sensors typically control the flow of calibration gases to the tonometer with electrical solenoid valves. In a typical gas flow control system for automated tonometry, the required gases are supplied from cylinders of calibration gases compressed under high pressure, through flow regulators and gas supply lines to a normally closed solenoid valve. For calibration of sensors which can sense multiple gas and/or pH conditions, the separate flow of two or more gas mixtures can be controlled by opening the valves as desired to allow flow through a manifold to the tonometer, to produce known gas concentrations and pH conditions in the tonometer.
The solenoid valves employed in such automatic tonometric systems generally include an electromagnet coupled to a metal plunger which has a resilient valve seat. The plunger is normally biased to a closed position by a spring, so that energization of the electromagnet acts to pull the plunger away from the valve seat, opening the valve to allow gas flow. Under ideal conditions such solenoid valves generally operate well in controlling gas flow over desired ranges of pressure and flow rates. However, incorrect seating of such valves, caused by such factors as degradation of the spring force over extended periods of time, or the lodging of metal or dust particles between the valve seat and the plunger, can allow gas leakage when the valve is supposed to be closed. In a situation where two or more gas compositions are separately introduced into a tonometer, failure of a valve to properly close can allow entry into the tonometer of a mixture of a first calibrating gas which is supposed to be turned off and a second calibrating gas which is turned on. Such an introduction of mixed calibration gases into the tonometer can cause significant errors in tonometric calibration which are not readily detectable.
It would therefore be desirable to provide a flow control system which can compensate for the failure of solenoid valves to operate properly in controlling gas flow to a tonometer for calibration of a blood gas or pH sensor. Although it would be possible to add backup solenoid valves for each main solenoid valve, it would be desirable to provide a common selector valve which can receive the output flow from two or more supply solenoid valves and select and direct one such output flow to the tonometer. It would also be desirable for the action of the supply solenoid valves and the common selector valve to be coordinated and controlled by a flow control device, to allow the tonometric calibration process to be automated. It would further be desirable that intermediate gas flow valves also select between gas flow of the supplied gas in an open position, and venting of the flow path to the common selector valve to a relatively lower pressure in a closed position to prevent introduction of any mixed calibration gases into the tonometer. It is also desirable to provide the gas flow control system with the capability of supplying three or more different gas compositions to the tonometer for calibration of a multiple sensor. The present invention meets these needs.